Balanced amplifier



Feb. 20, 1934. s. D. LAVOIE 1,948,303

BALANCED AMPLIFIER Filed April 2, 1931 Patented Feb. 20, 1934 UNITED STATES PATENT OFFICE 8 Claims.

This invention relates to an arrangement for amplifying alternating currents and more particularly to a balanced amplifying circuit.

An object of this invention is to provide an amplifying circuit which will give a greater undistorted signal strength, other things being equal, than has heretofore been possible.

A further object of this invention is to provide a balanced amplifying circuit having variable cathode resistances by which the characteristics of the separate tubes may be made to coincide throughout the operating range of the amplifier.

These and other objects will be apparent from the following specification and the accompanying drawing in which:

Fig. 1 is the circuit showing an application of the invention, and

Fig. 2 is a diagram showing the characteristic curves of the two tubes employed in the circuit of Fig. 1.

The tubes 1 and 2 have respectively anodes 3 and 4, cathodes 5 and 6, and grids '7 and 8. The grids 7 and 8 are connected to the opposite ends 9 and 10 of the secondary coils 11 and 12 of the input transformer 13. The other ends 14 and 15 of the secondary coils of the input transformer are connected through the grid bias batteries 16 and 17 respectively, which are in turn connected to the grounded center 18 of a common connection between the cathodes 5 and 6.

The cathodes 5 and 6 are energized by the secondary coils 19 and 20 respectively of a transformer 21, and the anodes 3 and 4 are connected to the opposite ends of the primary 22 of the output transformer 23. A suitable source of direct i current potential, such as a battery 24., is inserted between the center of the primary coil 22 and the ground.

In operation, the grid bias batteries 16 and 17 49 are so adjusted that the tubes 1 and 2 operate from substantially cut off position so that no plate current will flow when the alternating current voltage impressed upon the input transformer 13 falls to zero. Assuming a sign wave input, the positive one-half of a cycle is amplified by one tube and the negative half of the cycle is amplified by the other tube. In this manner, the two halves of the cycle are combined to produce an undistorted sign wave in the secondary winding of the output transformer.

Balanced amplifier circuits of this general type are well known but, as customarily adjusted and operated, have failed to function properly due to the fact that the two tubes of the balanced amplifier do not have identical operating characteristics. By individual adjustment of the variable grid bias sources 16, 1'7, it is possible to bring the characteristic curves of the two tubes into coincidence at one point, this adjustment being usually made when no signal is impressed upon the input transformer. Such static adjustment for one operating potential did not ensure a matching of the tube characteristics which will be maintained throughout the operating range of impressed signal voltages. In general, variations in tube characteristics made it impossible to secure an approximately exact balance of the tubes and therefore the maximum undistorted output of such amplifiers usually fell far short of the expected performance.

I have discovered that a substantially perfect balance may be attained even with tubes-which are not identical, when due regard is given to the dynamic as well as the static conditions affecting the amplifier operation.

In accordance with the present invention, adjustable resistances 25 and 26, respectively, are included in those portions of the cathode circuits of the tubes 1 and 2 which are between the com mon grounded point 18 and the respective cathodes 3, 4. As these resistances are common to the input and output circuits of the respective tubes, the flow of plate current produces a voltage drop which is available as a grid bias potential. The adjustable resistors therefore afford a means, in addition to the adjustable potential sources 16, 17 for determining the bias voltages applied to the individual tubes.

Referring now to Fig. 2, the curves A, B are static characteristic curves for two tubes of the same general type, the data from which the curves were plotted having been obtained under identical conditions as to operating potentials. If these tubes were employed in the circuit shown in Fig. 1, and the bias batteries were so adjusted as to impress the same negative bias, as represented by line (1, upon both tubes, the curves of Fig. 2 show that the plate current passed by tubes would not be equal. This condition may be corrected by adjusting the bias on tube B to the value b which corresponds to a plate current of the same magnitude as that established in tube A by a bias voltage a.

The effect of the individual adjustment of the bias voltages may be viewed as a lateral shifting of the characteristic curves A and B, to bring curve B into coincidence with curve A at a point on curve A corresponding to a fixed bias a. The curves are of substantially different slope and therefore this adjustment will not result in substantially identical amplification by the individual tubes.

This variation in slope is compensated, in accordance with this invention, by superposing upon the relatively fixed bias potentials established by sources 16, 17, additional bias potentials which are functions of the space current established in the respective tubes. To effect this second adjustment, the bias voltages are each made less negative by the same increment, which increment may be the assumed maximum value of a signal voltage to be impressed upon the amplifier. The cathode resistors 25, 26 are then adjusted to produce equal plate currents in the two tubes.

When so adjusted, the tubes will be accurately matched at two operating points and will be substantially matched at all other points throughout the range of impressed signal voltages. When this condition obtains, the maximum undistorted output is substantially equal to the theoretical output and the full advantages of this type of amplifier may be secured.

It will be apparent that other methods may be employed for balancing the amplifier under load conditions. The plate currents of the two tubes may be equalized, for example, by including in one or both plate circuits adjustable resistance of such magnitude that the effective plate voltage on the tube is a function of the plate current. With tubes having more than one grid, such as tetrode or pentode tubes, it is possible to introduce the balancing elements in the circuits of the additional electrodes. These and other methods for balancing the amplifier will be apparent to those familiar with the design and operation of thermionic amplifiers.

I claim:

1. In the operation of a balanced amplifier of the type including two thermionic tubes connected in phase opposition as regards an alternating current potential to be amplified, the method of compensating for variations in the operating characteristics of the two tubes which comprises adjusting thebias potentials on the grids of the respective tubes to balance the amplifier for input signal voltages of two different magnitudes.

2. The invention as set forth in claim 1, wherein one of the balancing adjustments is made under no load conditions and the other balancing adjustment is made automatically during the operation of the amplifier.

3. In the operation of a balanced amplifier of the type including two thermionic tubes connected in phase opposition as regards an alternating current potential to be amplified, and including cathode bias resistors and sources of relatively fixed bias potentials individual to the separate tubes, the method of balancing said amplifier which comprises adjusting the respective sources of bias potentials in the region of cut-off to secure the same plate current flow in both tubes, and adjusting the cathode resistors to secure the same plate current flow in both tubes when the voltage to be amplified is in the region of its maximum value.

4. In an amplifier, the combination with two electron discharge tubes each including a grid, means connecting said tubes for impressing alternating potentials in phase opposition upon the grids of both tubes for amplification thereby, and individual sources of potential for biasing the grids of said separate tubes, of adjustable cathode resistances for superposing upon the relatively fixed bias potentials additional bias potentials which vary automatically with the space current flow in the respective tubes.

5. In combination, two electron discharge tubes each including a grid and a cathode, means connecting said tubes for impressing an alternating potential in phase opposition upon the grids of the tubes for amplification thereby, sources of potential for biasing the grids of said tubes and adjustable cathode bias resistances individual to the separate tubes and in a common portion of the input and output circuits of the respective tubes.

6. In the operation of a balanced amplifier of the type including a pair of thermionic tubes each biased at cut-off and connected in phase opposition as regards an alternating current potential to be amplified, the method of compensating for variations in the operating characteristics of the two tubes which comprises the steps of adjusting the bias potentials on the grids of the respective tubes to balance the amplifier for input signal voltages of a plurality of different magnitudes sufficient to substantially cover the normal range of operation of the amplifier.

7. In combination, two electron discharge tubes each including a grid and a cathode, means connecting said tubes for impressing an alternating potential in phase opposition upon the grids of the tubes for amplification thereby, a source of potential for biasing the grids of said tubes at cut-off and adjustable cathode bias resistances individual to the separate tubes and in a common portion of the input and output circuits of the respective tubes.

8. A vacuum tube amplifier comprising, vacuum tubes arranged in push-pull relation each tube containing a control grid, a cathode and an anode, an anode circuit for each tube and means comprising an individual impedance in each anode circuit to control the potential difference between each grid and its associated cathode, and individual means connected between the grid and cathodes of each tube for maintaining said tubes in balanced relationship under no load conditions.

STEPHEN D. LAVOIE. 

